[clang] [libc++] Add builtin to clear padding bytes (prework for P0528R3) (PR #75371)

[Louis Dionne via cfe-commits]

================
@@ -0,0 +1,556 @@
+// RUN: mkdir -p %t
+// RUN: %clang++ %s -o %t/run
+// RUN: %t/run
----------------
ldionne wrote:

I'm not sure if this is going to work on all configurations, for example when they cross compile. Is there a precedent for these kinds of runtime tests in the codegen tests? Probably worth making sure that this is how it's done elsewhere.

https://github.com/llvm/llvm-project/pull/75371